This application is related to copending application having Ser. No. 08/024962, filed Mar. 2, 1983 now pending.
The invention relates to a low-template clathrasil, a process for its preparation and to the use of this material as an adsorbent.
Clathrasils belong to the class of porous tecto-silicates or porosils, which, according to H. Gies and B. Marler, Zeolites, 12 (1992), 42, can be described by the general formula ##STR1## in which A is a cation of charge y, T is a trivalent cation which is coordinated tetrahedrally by 4 O atoms, z is the number of additional O atoms for compensating lattice interruptions, AX are ion pairs and M are guest particles, and u, v and w are 0, 1, 2, with the upper limit being set by the size of the molecules able to be integrated within the structure. Examples of suitable cations A are alkali metal ions and alkaline earth metal ions, T can be, for example, an aluminum cation or boron cation, and, as X, a large number of anions such as, for example, OH, halogen, and the like, are suitable. The guest particles, which are also called templates, are incorporated in the interstices of the porosil structure and thus substantially affect t heir structure during synthesis of the porosils. The guest particles are in general neutral molecules, although, in particular in the case of charged porosil structures, charged guest particles can sometimes also be incorporated. In this case, the above formula (1) has to be modified accordingly, i.e., to A.sup.y.sub.x/y.spsb.-W/Q.sup.y where W is the number of charged guest molecules and Q is the number of charges.
According to the size and form of the interstices, porosils are divided into clathrasils and zeosils. While zeosils have cage- or channel-like pores whose size is such that the template molecules can be expelled from the pores under relatively mild conditions and, if desired, exchanged for other guest molecules, clathrasils have very small cage-like interstices whose openings are too small for a trapped molecule to leave the pore system. Clathrasils of the dodecasil 1 H, dodecasil 3 C, melanophlogite and nonasil type have pore openings of the less than 3 .ANG., the pore diameter being typically about 2.8 .ANG.. Such a narrow pore opening allows neither oxygen molecules to enter the pores nor CO.sub.2 molecules to leave, so that the template molecules cannot be removed from the pores even by a calcining step.
Hitherto, the clathrasils have been prepared by hydrothermal crystallization at, for example, 200.degree. C. from a silicate-containing solution to which template molecules, such as, for example, adamantylamine are added in excess, as a result of which virtually every sufficiently large interstice in the clathrasil crystals is occupied by a template molecule. The use of such a high template concentration has hitherto in general been considered necessary for obtaining defined and crystalline clathrasil structures of good morphology.
A disadvantage of the preparation processes previously described in the literature are the long synthesis times. Furthermore, in the customary processes, relatively large crystals having a size of between 150 and 500 .mu.m are produced, which is unnecessary or even undesirable for many applications. However, a particular disadvantage is that the clathrasils prepared by customary processes can be regarded as porous materials only to a limited extent, since virtually all available and sufficiently large interstices have already been occupied by template molecules during the synthesis. Since the template molecules can hardly be removed from the interstices, even under drastic conditions, it is virtually impossible to use the previously synthesised clathrasils as adsorbents.